1. Field of the Invention
This invention relates to a method of synthesizing metal oxides usable as, for instance, superconductors, semiconductors, magnetic materials and catalysts.
2. Description of the Prior Art
It is known that synthesis of certain metal oxides requires the oxygen partial pressure to be controlled to an extremely low level. Regarding synthesis of Sr2MoO4, for example, Lindblom et al. estimated from the oxidation-reduction potential of the metal oxide that the oxygen partial pressure must be maintained at an extremely low 10xe2x88x9221 atm. [B. Linblom and R. Rosen, Acta Chem Scand. A40, 452 (1986)]. Since such an extremely low oxygen partial pressure could not be achieved with an ordinary vacuum pump, they adopted a method comprising the steps of placing a mixture of Sr(OH)2 and MoO2 as a starting material and a mixture of SrO and Mo metallic powders as an oxygen partial pressure buffer side by side in a vacuum vessel and heating the vacuum vessel. They succeeded in obtaining Sr2MoO4 by this method.
When the oxygen partial pressure is lowered too much, however, metals in the starting material precipitate as simple substances that constitute impurities. The foregoing prior art method of controlling oxygen partial pressure with a mixture of SrO and Mo metallic powders therefore has a major drawback in that precipitation of Mo metal, Sr3Mo2O7 and other impurity phases on the starting material side makes production of a single-phase material impossible [U. Steiner and W. Reichelt, Z. Naturforsch. 53b, 110 (1998)].
An object of the present invention is to provide a method of synthesizing metal oxides that utilize a rationally selected oxygen partial pressure control agent to achieve oxygen partial pressure control even in an extremely low oxygen partial pressure region of 10xe2x88x9221 atm. and thus to enable synthesis of a target metal oxide without precipitation of simple metal as impurity on the starting material side.
The present invention achieves this object by providing a method of synthesizing metal oxides that, when synthesizing an oxide of a metal A, controls oxygen partial pressure by using an oxygen partial pressure control agent containing an oxide of a metal B capable of assuming multiple valence numbers, which method satisfies an ionization tendency relationship of A1+ less than Bj+ less than A0, where i is a valence number of the metal A and j is a valence number of the metal B.
An example of synthesizing Sr2MoO4 using Ti2O3 as the oxygen partial pressure control agent will be described. In this case, metal A=Mo, metal B=Ti, and the Ti3+ (Bj+) ions in the oxygen partial pressure control agent Ti2O3 have an ionization tendency just midway between the Mo4+ (A1+) ions as in Sr2MoO4 and Mo (A0) simple metal, that is, the relationship of Mo4+ less than Ti3+ less than Mo holds. The first half of the relation, Mo4+ less than Ti3+, ensures that residual oxygen gas in the vacuum vessel is absorbed by Ti2O3 to produce the extremely low oxygen partial pressure of 10xe2x88x9221 atm. necessary for Sr2MoO4 formation. At the same time, no liberation of Mo metal occurs since Ti3+ less than Mo. Pure Sr2MoO4 can therefore be synthesized.
The above and other objects and features of the invention will become apparent from the description made with reference to the accompanying drawings.